Nonemissive electrode and method of manufacturing



Feb. 12, 1952 K. E. EVERETT NONEIM'ISSIVE ELECTRODE AND METHOD OF MANUFACTURING Filed Aug. '7, 1947 E R M W E D m 0 m m -w 6 z v 0- w o w. w m w m a v 3 (12/0 o/ss/PA TION (WATTS) lNVENTOR KENNETH E. EVERETT ATTORNEY Patented Feb. 12, 1952 NONEMISSIVE ELECTRODE AND METHOD OF MANUFACTURING Kenneth-Edward Everett, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application August 7, 1947, Serial No. 766,980.

In Great Britain March 15, 1946 Section 1, Public Law 690, August 8, 1946 Patent expires March 15, 1966 This invention relates to improved electrode and a method of manufacture thereof for electron discharge devices whereby electronemission from said electrodes is inhibited. The method is of particular application to the grids of high power tubes. In high power vacuum tubes such as transmitting valves it is common for the control grid to dissipate considerable power. Under such conditions difllculties have arisen in the past through electron emission from the surface of the electrode. Such emission is objectionable in most valves, although the problem is more acute in those of the high power class in which it is intended that grid current shall flow. The electron emission arises from two sourcesthermi- Ohio or primary emission due to the heating of the electrode and secondary emission due to bombardment, ionic or electronic, of the atoms in the surface layer of the electrode.

It has been known for some time that primary emission may be reduced by applying to the grid coating of carbon, usually inthe form of lamp black. More recently, it has been found that a coating of zirconium is very effective in preventing secondary emission, more so than carbon. A carbon coating however, is more effective in preventing primary emission than one of zirconium alone. Although both types of coating effect a considerable reduction in primary emission, the reduction so obtained is not as great as would be desired. Since thermionic emission is largely a surface phenomenon it might be expected that no advantage would arise in applying superimposed coatings of carbon and zirconium. Applicant has found, however, that this is not the case and that, particularly if a double coating be applied in the manner to be described, a double coating is, out of all proportion, more effective in reducing primary emission. For a better understanding of the present invention reference may be had to the following description taken in connection with the accompanying drawing, in which,

Fig. 1 is a graph showing the emissivity of an electrode under various conditions of coatings including that type by my invention,

Fig. 2 is a perspective view of a portion of an electrode partly in section, made in accordance with this invention.

InFig. 1 of the attached drawing, curve A shows the measured relationship between the primary emission from a particular grid electrode and the power dissipation therein. Curve B shows the improvement obtained when a similar grid was coated with zirconium and curve C for a third 8 Claims. 01. 313-407) 2 similar electrode coated with lamp black, 'while curve D shows the result obtained with'a double coating in accordance with the present invention. In all the cases, B, C and D, the weight of total coating was the same.

It should be noted that it is already known in the art to coat an electrode with carbon and then topaint on a zirconium coating for the purpose of removing residual gases from the evacuated valve by absorption of the gases by the highly oxidizable zirconium. Carbon is here used merely to bind the metallic coating to the electrode structure. For the purpose of reducing primary and secondary emission I have found it not only more effective, but a more convenient process to apply the carbon on top of the zirconium coating.

According to the present invention there is provided a method of manufacturing an electrode for an electron discharge device comprising coating the electrode with zirconium and then coating the coated electrode with carbon.

For application to the grid of high power valves, I have found it preferable to apply a coating of zirconium to the clean metal of the grid by means of electrophoresis, the process being continued until the grid is seen to be just covered uniformly with zirconium. The zirconium-coated grid is then sprayed with lamp black until it is seen that a uniform coating which just covers the previous coating has been obtained. In order to obtain a uniform product there may be selected any suitable air pressure applied to the spraying pistol together with a desired number and speed of passes of the spray over the article under treatment. Finally, the coated grid is fixed in vacuo at a very high temperature-about 1600 C. being suitable. Fig. 2 shows an electrode wire coated with zirconium and then with a stratifiedreduction of primary emission to be obtained is not understood but it is thought that during heat treatment the surface is changed so that it is no longer carbon, or else that a semi-conducting layer is formed between the two coatings of zirconium and carbon.

What is claimed is:

1. The method of .manufacturing an electrode for an electron discharge device comprising the steps of causing the migration of suspended parti- 3 cles of zirconium exclusively to said electrode to coat same under the influence oi an electric field and then applying a coating of carbon thereover to reduce electron emission from said electrode when said electron discharge device is in operation.

2. Method of manufacturing an electrode for an electron discharge device comprising coating the electrode exclusively with zirconium, then applying a coating or carbon and firing the coated electrode in vacuo so as to reduce electron emission from said electrode when said electron discharge device is in operation.

3. Method of manufacturing an electrode for an electron discharge device according to claim 2, in which said first coating step comprises applyin zirconium to said electrode by electrophoresis, said second coating step comprises spraying carbon onto said coated electrode and said firing step comprises finally firing the coated electrode in a vacuum at a temperature 01 substantially 1600" C.

4. An electrode for an electron discharge device comprising a base structure having a portion from which "electron emission is to be inhibited, a first coating of zirconium exclusively upon said portion, and a coating of carbon upon said first mentioned coating, the proportion by weight of zirconium and carbon applied to said electrode being of the order of 8 to 1, respectively 5. An electrode for an electron discharge device comprising a base structure having a stratified coating of carbon deposited upon a prior exclusive coating of zirconium to inhibit electron emission from said electrode.

8. An electrode for an electron discharge device comprising a portion from which electron emission is to be prevented, a layer of zirconium exclusively coated upon said portion, and a stratifled coating of carbon upon said first-mentioned yer.

7. An electrode for an electron discharge device comprising a portion havinga stratifled coating to inhibit electron emission therefrom, said coating comprising an exclusive layer of zirconium treated with an overlying stratified coating of carbon. v

8. A non-emissive composite electrode structure that includes a metal base bearing a coating of zirconium exclusively with a superficial stratifled coating of carbon overlying said zirconium.

KENNETH EDWARD EVERETT.

REFERENCES CITED The following references are of record in the file oi this patent:

I UNITED STATES PATENTS Number 

8. A NON-EMISSIVE COMPOSITE ELECTRODE STRUCTURE THAT INCLUDES A METAL BASE BEARING A COATING OF ZIRCONIUM EXCLUSIVELY WITH A SUPERFICIAL STRATIFIED COATING OF CARBON OVERLYING SAID ZIRCONIUM. 